Back pain in the lower, or lumbar, region of the back is common. In many cases, the cause of back pain is unknown. The human back is a complicated structure including bones, muscles, ligaments, tendons, nerves and other structures, which together form the spinal stabilization system. The spinal stabilization system may be conceptualized to include three subsystems: 1) the spinal column, which provides intrinsic mechanical stability; 2) the spinal muscles, which surround the spinal column and provide dynamic mechanical stability; and 3) the neuromotor control unit, which evaluates and determines requirements for stability via a coordinated muscle response. In a properly functioning system, neuromotor control unit sensors present in the connective tissue of the spinal column and the muscle spindles of the spinal muscles each transmit signals via nerves to the motor cortex of the brain to provide information such as the force a muscle is exerting or the position of a joint. The motor cortex uses signals from the body's neuromotor control unit sensors to form a sense of the body's position in space. This sense is referred to as proprioception. The motor cortex of the brain returns signals to the spinal muscles to control the spine's position in space. Thus, in patients with a functional stabilization system, the three subsystems work together to form a feedback loop that provides mechanical stability to the spine. It is applicant's realization that lower back pain often results from dysfunction of these subsystems and disruption of the feedback loop.
Some cases of back pain are caused by abnormal mechanics of the spinal column. The spinal column consists of vertebrae and ligaments, e.g. spinal ligaments, disc annulus, and facet capsules. Degenerative changes to these structures, injury of the ligaments, acute trauma, or repetitive microtrauma may lead to back pain via inflammation, biochemical and nutritional changes, immunological factors, changes in the structure or material of the endplates or discs, and pathology of neural structures.
It is believed that in some patients with back pain, the spinal stabilization system is dysfunctional. Under normal circumstances, mechanoreceptors present in the ligaments, facet capsules, disc annulus, and other connective tissues generate signals describing spinal posture, motions, and loads. These signals provide information to the neuromuscular control unit, which generates muscle response patterns to activate and coordinate the spinal muscles to provide dynamic mechanical stability. The neuromuscular control unit produces a muscle response pattern based upon several factors, including the need for spinal stability, postural control, balance, and stress reduction on various spinal components. If the spinal column structure is compromised, for example, due to injury, degeneration, or viscoelastic creep, then muscular stability must be adjusted to compensate and maintain spinal stability. However, ligament injury, soft tissue fatigue, viscoelastic creep, and other connective tissue injuries may cause mechanoreceptors to produce corrupted signals about vertebral position, motion, or loads, leading to an inappropriate muscle response. In addition, muscles themselves may be injured, fatigued, atrophied, or lose their strength, thus aggravating dysfunction of the spinal stabilization system. Moreover, muscles may disrupt the spinal stabilization system by going into spasm, contracting when they should remain inactive, developing trigger points, or contracting out of sequence with other muscles. Such muscle dysfunction may cause muscle spindle mechanoreceptors to send abnormal signals to the motor cortex, which further may compromise normal muscle activation patterns via the feedback loops.
Through such mechanisms, disruptions to the spinal stabilization system can result in spine instability, which can lead to low back pain. In particular, spine instability can result in the generation of high loads on spinal structures when the spine moves beyond its neutral zone. The neutral zone is a range of intervertebral motion, measured from a neutral position, within which spinal motion is produced with a minimal internal resistance. High loads can lead to inflammation, disc degeneration, facet joint degeneration, and muscle fatigue. Since the endplates and annulus have a rich nerve supply, it is believed that abnormally high loads on such structures, resulting from spine instability, may be a common cause of pain. Load transmission to the facet joints also may increase with degenerative disc disease, leading to facet arthritis and facet joint pain.
A need exists for improving spine stability in many patients suffering from lower back pain. It is applicant's hypothesis that repetitive and episodic contraction of the local muscle system of the back may generate afferent signals to the brain capable of reactivating or awakening the spinal stabilization system, thereby stabilizing the spine and reducing pain.
The local muscle system includes deep muscles, and portions of some muscles that have their origin or insertion on the vertebrae. These local muscles control the stiffness and intervertebral relationship of the spinal segments. They provide an efficient mechanism to fine-tune the control of intervertebral motion. The lumbar multifidus, with its vertebra-to-vertebra attachments, is an example of a muscle of the local muscle system.
The multifidus is the largest and most medial of the lumbar back muscles. It has a complex structure with repeating series of fascicles stemming from the laminae and spinous processes of the vertebrae, which exhibit a consistent pattern of attachments caudally. These fascicles are arranged in five overlapping groups such that each of the five lumbar vertebrae gives rise to one of these groups. At each segmental level, a fascicle arises from the base and caudolateral edge of the spinous process, and several fascicles arise, by way of a common tendon, from the caudal tip of the spinous process. Although confluent with one another at their origin, the fascicles in each group diverge caudally to assume separate attachments to the mamillary processes, the iliac crest, and the sacrum. Some of the deep fibers of the fascicles that attach to the mamillary processes attach to the capsules of the facet joints next to the mamillary processes. The fascicles arriving from the spinous process of a given vertebra are innervated by the medial branch of the dorsal ramus nerve that issues from below that vertebra.
The lumbar multifidus and other skeletal muscles consist of a number of specialized elongated cells mechanically coupled together. A nerve fiber connects to the muscle cells at a region called the end plate. The combination of the muscle cell or group of cells and the nerve fiber that innervates it is called a motor unit. Motor units come in different sizes, with larger motor units producing greater force than smaller motor units given equal stimulation. An electrical signal transmitted to a nerve will travel down the nerve fiber and cause depolarization of the cell wall of the muscle fiber, thereby triggering biochemical processes inside the muscle cell that generate a twitch of contraction and resultant force generation.
Nerves to skeletal muscles generally include a mix of motor nerves and sensory nerves. Motor nerves are efferent nerves, which carry electrical signals from the brain to cause an action in a muscle, and sensory nerves are afferent nerves, carrying signals from remote structures to the brain to provide information to the brain.
External electrical stimulation for causing muscle contraction has been known since Galvani observed such contraction in frogs in 1791. Over time, it became known that the most energy efficient way to apply electrical stimulation to cause a muscle contraction is to stimulate the nerve fiber of the motor unit because the energy required to stimulate a nerve fiber to elicit contraction is about 1000 times less than required to stimulate a muscle to elicit contraction.
If an electrical stimulation electrode is placed on or adjacent to the nerve that supplies the muscle, then a single electrical pulse will cause a single contraction of the muscle referred to as a twitch. The force in the muscle rises rapidly and decays more slowly to zero. The amount of muscle that contracts, and hence, the force of contraction, in the twitch is determined primarily by the number of motor units stimulated.
If additional stimulation pulses are applied, additional twitches are produced. If the rate of stimulation is such that a new stimulation pulse is presented before the prior twitch has decayed, then the new twitch will be largely superimposed on the prior, producing a summation of force. As the stimulation rate is increased, this summation of force is such that the twitches blend together to generate a smooth contraction. The stimulation frequency at which the force production transitions from intermittent (rapid twitching) to smooth contraction is often referred to as the fusion frequency. Stimulation at a rate at or above the fusion frequency leads to smooth force generation. In general terms, stimulation at a rate significantly higher than the fusion frequency has minimal effect on the strength or nature of contraction and may, in fact, have an adverse impact on fatigue of the muscle. Stimulation at a frequency higher than necessary to achieve the desired (e.g., maximum) force is energy inefficient, which is an important consideration for an implantable device.
Functional electrical stimulation (FES) is the application of electrical stimulation to cause muscle contraction to re-animate limbs following damage to the nervous system such as with stroke or spinal cord injury. FES has been the subject of much prior art and scientific publications. In FES, the goal generally is to bypass the damaged nervous system and provide electrical stimulation to nerves or muscles directly, which simulates the action of the nervous system. One lofty goal of FES is to enable paralyzed people to walk again, and that requires the coordinated action of many muscles activating several joints. In patients with spinal cord injury, the sensory nervous system is usually damaged as well as the motor system, and thus the afflicted person loses proprioception of what the muscle and limbs are doing. FES systems often seek to reproduce or simulate the damaged proprioceptive system with other sensors attached to a joint or muscle.
Neuromuscular Electrical Stimulation (NMES) is a subset of the general field of electrical stimulation for muscle contraction, as it is generally applied to nerves and muscles which are anatomically intact but malfunctioning in a different way. NMES may be delivered via an external system or, in some applications, via an implanted system.
NMES via externally applied skin electrodes has been used to rehabilitate skeletal muscles after injury or surgery to an associated joint. This approach is commonly used to aid in the rehabilitation of the quadriceps muscle of the leg after knee surgery. Electrical stimulation is known to not only improve the strength and endurance of the muscle, but also to restore malfunctioning motor control to a muscle. See, e.g., Gondin et al., “Electromyostimulation Training Effects on Neural Drive and Muscle Architecture”, Medicine & Science in Sports & Exercise 37, No. 8, pp. 1291-99 (August 2005).
An implanted NMES system has been used to treat incontinence by stimulating nerves that supply the urinary or anal sphincter muscles. For example, U.S. Pat. No. 5,199,430 to Fang describes an implantable electronic apparatus for assisting the urinary sphincter to relax.
For rehabilitation of anatomically intact (i.e., functionally disordered) neuromuscular systems, the primary goal is to restore normal functioning of the neuromuscular system. One application for an implanted NMES system is to restore normal functioning of the spinal stabilization system in order to improve spine stability in patients suffering from lower back pain. Such an application is described in U.S. Pat. Nos. 8,428,728 and 8,606,358 to Sachs and U.S. Application Publication No. 2011/0224665 to Crosby, each of which is incorporated herein by reference in its entirety. These references describe implanted electrical stimulation devices designed to restore neural drive and rehabilitate local muscles of the back, such as the multifidus muscle, to improve stability of the spine. It is theorized here that providing appropriate electrical stimulations to the multifidus muscle using an implanted NMES system to generate repetitive and episodic contractions of the multifidus muscle may reactivate the feedback loop and spinal stabilization system over time.
Another form of stimulation therapy is trans-cranial magnetic stimulation (TMS), which also may be used to activate skeletal muscles. In TMS, a time varying magnetic field is generated to induce an electrical current. Applying such a magnetic field with a coil positioned over a patient's skull can induce an electrical current in the patient's brain tissue. This technique has been used to stimulate portions of the motor cortex by applying and focusing a magnetic field over certain regions of the brain, primarily in the motor cortex. A patient's response to TMS pulses can be observed as a muscle twitch or as an electrical signal such as an electromyogram (EMG). TMS has been used to reactivate the quadriceps muscle following loss of volitional quadriceps activation resulting from meniscectomy.
One of the challenges of stimulation therapies such as NMES and TMS is monitoring to ensure the stimulation device is positioned properly, applying appropriate levels of stimulation, and resulting in a positive therapeutic effect. Monitoring can be especially challenging for deep muscles such as the deep fascicles of the lumbar multifidus, which are too deeply positioned for contractions to be reliably observed visually. A related challenge of NMES and TMS for rehabilitation of skeletal muscles is to diagnose when the therapy has been successful and may be discontinued. This is particularly important with patients who cannot communicate, e.g., young children, or patients who do not want to communicate, e.g., malingerers who may be motivated for the therapy to not be successful as it would result in loss of worker's compensation insurance.
It would therefore be desirable to provide a system and method to objectively monitor progress and diagnose when stimulation therapy of a skeletal muscle has been successful. To further research and the development of future therapies, it would also be desirable to provide a system and method that enable mapping at the various areas of the motor cortex and enable generation and display of motor cortex representations of the muscles. Accordingly, it would be advantageous to provide a system and method that enable controlled monitoring of muscle responses resulting from various motor cortex stimulations. Such muscle responses may result in recordable signals, such as evoked potentials.
An evoked potential is an electrical signal recorded from a part of the body, which results from the presentation of a stimulus to a portion of the body. Evoked potentials include, for example, somatosensory evoked potentials (SSEPs), visual evoked potentials (VEPs), motor evoked potentials (MEPs), and brain stem auditory evoked potentials (BAEPs). SSEPs consist of a series of electrical waves that reflect sequential activation of neural structures in the somatosensory pathways. SSEPs can be measured at the cortex of the brain or at various sites along the somatosensory pathway, including at peripheral nerves. SSEPs can be triggered with electrical stimulation along the somatosensory pathway, for example, at a peripheral nerve. SSEPs can also be triggered by mechanical stimulation near a peripheral nerve.
Evoked potentials are currently used as a measure of nerve functionality in some clinical procedures. Current clinical uses of evoked potential testing include measuring nerve signal conduction velocity, which can be an important diagnostic tool for diseases of the nervous system, such as multiple sclerosis, and verifying spinal cord functioning during spine surgery, as described in U.S. Pat. No. 8,016,846 to McFarlin et al. and U.S. Pat. No. 7,981,144 to Geist et al. Evoked potentials can also be used on a temporary basis as an aid to placing electrodes in or near the nervous system, for example, at the dorsal root ganglion, as described in U.S. Pat. No. 7,337,006 to Kim et al. A variety of techniques have been developed for the analysis of evoked potentials, for example, the techniques described in U.S. Pat. No. 8,391,966 to Luo et al., U.S. Pat. No. 5,638,825 to Fukuzumi et al., and U.S. Pat. No. 8,498,697 to Yong et al.
Compared to other biological signals, many types of evoked potentials are quite small. Often, in clinical situations, the small size of an evoked potential is not visible in the raw data when a single stimulus is applied. To extract the electrical signal of interest from the background noise, the technique of signal averaging is employed. Signal averaging can be spatial, temporal, or some combination (i.e., spatio-temporal averaging). In spatial averaging, a mathematical combination of signals are collected over a region of space in response to a stimulus. In temporal averaging, a mathematical combination of signals are synchronized in time in response to a stimulus. With temporal averaging, the electrical signals recorded following the stimulus are sampled using an analog-to-digital converter, then the time series of the samples is added together and divided by the number of samples to preserve scaling. The time series is synchronized with the stimulus event. In this manner, the background signal, which is asynchronous to the stimulus, tends towards its mean of zero, and the evoked potential average tends to a useful value above the background noise. The signal-to-noise ratio improves with the square root of the number of responses that are averaged. As will be appreciated by one skilled in the art, the specific combination of filtering parameters, sampling frequency, and number of scans to be averaged is determined by the nature of the evoked potential to be measured.
In the description provided herein, the term “evoked potentials” refers to electrical signals. There are other “evoked response” signals generated in response to a stimulus, such as force generation or movement of a muscle in response to electrical stimulation and motion of the eyes (saccade) in response to a visual stimulus.
It would be desirable to provide a system or method to detect and measure evoked potentials and other evoked responses to objectively monitor progress, optimize treatment, and diagnose when rehabilitation of a skeletal muscle has been attained.
It would be desirable to provide a system or method for monitoring and recording progress of NMES or TMS for rehabilitation of the lumbar multifidus muscle.
It further would be desirable to provide a system or method that provides data needed to adjust the operating parameters of an NMES or TMS system based on measurements of muscle performance, thereby continually optimizing the stimulation system.
It would also be desirable to monitor the effects of a stimulation system on a tissue's electrical activity, for example, to confirm applicant's hypothesis that repetitive and episodic contraction of the local muscle system of the back generates afferent signals to the brain capable of reactivating or awakening the spinal stabilization system. It would thus be desirable to provide a system and/or method capable of detecting and recording signals generated by a patient's body in response to repetitive and episodic stimulations to, and contraction of, the local muscle system of the back.